This invention relates generally to the formation of drum shells consisting of wood, such as plywood; and more particularly the invention concerns cooling of shells after their formation in annular configuration, under heat and pressure.
Multi ply drum shells are typically made of plywood consisting of single ply, 2 ply and 3 ply cross laminate lay ups. Drum shells may consist of a combination of those factors.
When such plies are rolled together with glue (or any type of adhesive) and inserted into a mold, there are two elements that work together to form the shell-pressure and heat. The mold (also known as a heating device or drum shell machine) typically has an outer platen and an inner platen, which work via hydraulics, as well as serving as heating elements. The outer platen closes to establish the outside diameter of the shell. The inner platens, provided with a tapered pin to displace them, spreads the inner platens toward the outer platens, thus trapping the plywood (shell) in between the inner and outer platens at roughly 200 degrees Fahrenheit, heat and pressure working together to form the inner diameter of the shell.
The shells typically remain in this heating device under about 2,500 psi. of hydraulic pressure for 5.5 (five and one-half) minutes. Adhesives that are used to bond the plywood typically activate under heat and pressure. Substantially constant pressure is an important element, besides the heating process. After the shell has been properly heated and molded, the pressure from the inner platen and then from the outer platen is relieved, releasing the shell, which is removed from the heating device. Typically, at this stage the drum shell is then placed on a cold cement floor and another shell is loaded into heater, for treatment.
It has been found that the cold cement floor cools the shell from the floor, such cooling traveling up the shell to create a colder and cured shell by the time it reaches the top of shell. During that process, the annular shape of the drum shell can and does distort leading to problems in use of the final drum.
It is vital for drums shell to be round and hard. The roundness helps the tuning process of the drum i.e. having the drum head centered, top and bottom, to achieve maximum tunability, with bearing edges for drum heads on a flat and even plane. The hardness of the shell is necessary to allow the hollow interior chamber to vibrate freely and with continuity in use. Between these objectives lies the possibility for a successful sounding tuned drum shell. In the past it was found that the success of realizing both these elements or characteristics was only achieved by hit and miss, hoping that the elements would come together naturally. However, through many years of experience and research it was discovered that the failure rate of realizing both of these two elements is fairly high i.e.; dry whether, cold-damp weather, heat, moisture, wind as well as many other influential elements contributed to the failure or rejection rates of drum shells. There is need for a new process that virtually ensures a consistent manufacturing process to consistently realize these objectives.
All drums are exposed to extreme variations in temperature due to a wide variety of elements. Most commonly these include exposure to a hot concert stage, then packing into a cold truck, and then returning to a hot, dry or moist environment. Due to such repeated exposures the potential for the drum shell to go out of round is quite high, absent starting off with as perfectly round a drum shell as possible, ultimately threatening the integrity of the sound.
There is need to prevent such uneven cooling of the heat and pressure formed plywood shells, in an efficient reliable manner. Also, there is need to significantly speed up the over-all process of heating and cooling plywood drum shells. Further, there is need to prevent distortion of the plywood shell as it cools.